The invention is about a process for coloring (printing) of web-like objects, especially ski coverings, from thermo-plastics such as olefins, especially high and low pressure polyethylene, where the side of the covering to be printed on is brought in contact with a print carrier where a fusion deposit is accomplished by means of heat, where the print carrier first has been covered with sublimation type color pigments and then with diffusion-type color pigments, as well as an apparatus for the implementation of such a process and of such a printed track-like object.
As is generally known, so called multiple layer skis have been manufactured for some time by glueing several layers together (see for example DE-OS No. 2 804 943) where these layers respectively form a lower, central, and upper structure.
The central structure generally consists of a wood or hard foam core, which is sideways pressed in with duro-plastic synthetic resin;
The upper structure generally consists of an aluminum sheet and/or laminate of fiberglass reinforced plastic glued to the core, on which a plastic foil covering is applied, and which is decorated in subsequent working processes;
The lower structure on the other hand consists of an aluminum sheet or of a layer of fiberglass reinforced plastic, which is glued on one side to the core and which on the other side (possibly through several more layers) is glued with a proper runner layer, which normally is a transparent synthetic resin covering of 1-2 mm thickness and preferrably of polyethylene, namely high or low pressure polyethylene, or polytetrafluorethylene (Teflon, Trademark).
Normally the inner side of the runner layer (or also of the top covering) is printed, especially with a ski manufacturer, type designations, etc.; this printing is visible through the transparent layer or covering.
Until now, essentially only high and low pressure polyethylene ski coverings were used.
The decorations, especially the printing of such layers and further work processes has been done until now very expensively (see DE-OS No. 2 804 943) and still results often in bad quality:
It is done piecewise, that is not continuously or endlessly, namely through screen pressure (from patterns, lettering or the like) with epoxy pressure colors, where in addition the layer has to be treated before printing such as thermally (flame contact), electrically (Corona-discharge) or chemically (etching), since otherwise the (epoxy) print color would not adhere;
After the printing process a drying time of 10-20 hours is necessary after which the printed and dried layer is treated with an opaque (colored) epoxy-containing casting material (an opaque covering lacquer) including a hardener, in order to produce a background color layer for the transparent covering;
this casting material layer is then glued to a layer of fiberglass reinforced plastic, however only after several hours after which the solvent material has completely left the casting material which means after the mass has hardened.
In spite of this fabrication process, which is especially demanding, it has not been possible to secure a printing quality of good continuity on the completed ski.
The following deficiencies occur rather late, namely 3 to 10 months after manufacture of the ski:
insufficient adhesion preparation of the covering before printing;
Attack of the printed picture by the adhesive so that color tones are changing, the contours of the printed picture become diffuse or cracks develop in the printed arrangement;
insufficient elasticity of the casting material relative the covering or insufficient adhesion of the opaque casting material on the polyethylene of the covering so that the separation of the casting material manifests itself in the form of light spots on the runner side;
insufficient evaporation of the solvent from the opaque casting mass which causes adhesion deficiencies because of formation of bubbles between the casting mass and for example the polyethylene of the covering, thus producing bad looking light spots. Similar difficulties as with the runner covering occur when a corresponding top cover which is to be decorated is used as top covering layer.
These difficulties as of this day (inspite of at least 10 years experience) still pose a series problem, sincethe cause of the deficiencies has not been researched in detail, so that there results often claims from the ski buyer against the ski manufacturer.
Finally, the best obtainable printing to quality is very limited, in spite of all the effort, causing relatively costly structured printing patterns, that is, the printing of photographs with half tones is practically impossible.
The transfer printing process in accordance with DE-OS No. 2 642 350 is relatively demanding and very difficult to control, where the surface to be printed on is first covered with a thermoplastic synthetic resin layer such as polyurethane or polyester which can be printed without difficulty, this being done to avoid conveying by migration.
It is taught in DE-OS No. 2 731 121 to print on polyethylene by bringing the side of the form member to be printed on in contact with a print carrier such as paper print carrier, with applying heat to obtain a fusion deposit where there has been applied to the print carrier a printing pattern of a dye which transfers by sublimation and then diffusion. Preferred temperature 160.degree.-220.degree. C. is given. As a "sufficient" contact-pressure duration of the print carrier against the surface to be printed on, 5-30 seconds is suggested. However, under these conditions given in DE-OS No. 2 731 121 the same migration problems arise which one tries to avoid in accordance with DE-OS No. 2 642 350 by separately applying layers to the side to be printed on.
Obviously, the migration problems occur because of an excessively long contact pressure time under high heat whereas on the other hand a relatively high contact pressure is required to obtain sufficient penetration of the dye stuff into the form member.
When reworking the process as described in DE-OS No. 2 731 121, one finds that such great thermal stress (stress peaks) develop in the form member due to the fusion deposit of the surface to be printed on, which makes it impossible to print on relatively thin walls, for example track-like objects, especially ski laminations. An extreme stress decrease occurs in the area of the boundary line between the fused pressure side and the lower side which still is in a solid state, where the decrease of tension results in warpage of the printed layer during cooling.
Besides the warpage problem so far described, there occurs with "form free" printing of thin walled objects the problem that the paper print carrier when pressed on longer than one second becomes partly unremovable from the fused-on synthetic resin, when using thin paper print carriers as employed in the printing of textiles. The printed surface then is essentially unusuable.
Finally, it is another advantage of the known solution in accordance with DE-OS No. 2 731 121 that "an outer second heat application" is required for printing. The form member to be printed on must be fused on the pressure side by means of a heating plate or the like.
Therefore, the process known from DE-OS No. 2 731 121 because of reasons given above is not suited for coloring or printing of web-like objects, especially ski coverings.